Short description of error
It seems that blender property registration using the ":" does not work for classes that are not blender specific, ie a simple 'object' class.
Blender cannot find the property registered this way, it will work fine if registered with an '=', however.
Exact steps for others to reproduce the error
Paste the following script in a text area, and run it:
import bpy class TestClass(object): test: bpy.props.BoolProperty() testclass = TestClass() print(testclass.test)
Blender will complain:
AttributeError: 'TestClass' object has no attribute 'test'